1. Technical Field
The invention relates to power supply circuit arrangements for electrical equipment and is especially applicable to power supply circuit arrangements which allow the equipment to be switched between an operational mode and a standby mode.
2. Background Art
It is known to provide electrical equipment, such as a television receiver, with a separate electronic control circuit which is connected across the input terminals of the equipment, upstream of a main on/off switch. The control circuit controls the switch to control application of the power supply voltage to a transformer in the equipment's main power supply circuit. The electronic control circuit has its own step-down transformer and AC-DC converter. In addition, it has a detector for detecting a control signal, perhaps an infra-red signal from a remote control. In the standby mode, the main on/off switch is open, but the control circuit is still energized. In response to a control signal, the control circuit closes the main on/off switch and energizes the main power supply circuit.
For safety reasons, it is desirable for the control circuit to be isolated from the main supply voltage when the equipment is in the standby mode. While such known control circuits operated by an infra-red, ultrasonic or other such electrically-isolated remote control device provide isolation, it is sometimes desirable for the control signal path to be supplied via a direct electrical connection. For example, some high fidelity audio systems and so-called "surround sound" systems employ a sub-woofer which has its own audio power amplifier. It is desirable for the sub-woofer power amplifier to be connected to a nearby power outlet and to receive only the audio signal from the audio preamplifier. When a control circuit in the sub-woofer detects receipt of an audio signal via the audio channel, it will operate a switch to connect the power supply to the sub-woofer power amplifier. It is desirable for the control circuit to provide a degree of electrical isolation between the power amplifier and the preamplifier when in the standby mode.
An object of the present invention is to eliminate, or at least mitigate, the disadvantages of known power supply arrangements and to provide an improved power supply arrangement for electrical equipment which permits switching between operational and standby modes while providing a desired degree of electrical isolation and/or reduced power consumption.